The present invention relates to a novel method of heat treating lead compositions as well as novel and improved materials produced by such treatment. The present invention finds particular application in the field of lead-containing battery grids, and provides solutions to a number of disparate problems heretofore encountered in this field.
A first problem involves the manufacture of lead battery grids which contain one or more additional elements in order to reduce weight, increase conductivity and/or increase strength. For example, various compositions of lead and aluminum have been tried since aluminum is both lighter and more conductive than lead. In this regard, a major goal of lead-acid battery manufacturers is to improve the efficiency of their products. One way of accomplishing this is to reduce the weight of their products and to lower the internal resistance of their batteries. In the case of electric vehicles (for example, golf carts), lighter batteries would extend the driving range and more conductive battery grids would improve the rate of recharging (especially in the case of tall batteries).
The following table, containing data represented in FIGS. 1 and 2, illustrates the reduction of density and resistivity in lead grids by the substitution of aluminum.
______________________________________ Density Resistivity Lead Aluminum % Re- % Re- wt. % wt. % Vol. % g/cm.sup.3 duction .mu.ohm-cm duction ______________________________________ 100 0 0 11.4 0.0 20.7 0.0 95.4 4.6 8 10.7 6.1 19.2 7.0 88 12 20 9.7 15.3 17.1 17.4 81 19 30 8.8 22.9 15.3 26.1 ______________________________________
Both density and resistivity follow the law of mixtures, that is, they are proportional to the volume percentage substitution of aluminum in the lead grid. Thus, for example, a 30% by volume substitution of aluminum (19% by weight) in a lead battery grid will result in a 22.9% weight reduction and a resistivity reduction of 26.1%.
On the other hand, lead and aluminum are substantially immiscible and they have widely varying melting points (lead=327.5.degree. C.; aluminum=660.37.degree. C.) and boiling points (lead=1740.degree. C.; aluminum=2467.degree. C.). It is well known that when metals and alloys are heated and cooled slowly they tend to recrystallize and develop larger-grain structures. This generally lowers the hardness, tensile strength, and corrosion resistance of such metals and alloys. During the recrystallization process, foreign elements in the lead tend to segregate at the grain boundaries and promote intergranular corrosion in battery acid electrolytes.
Therefore, considerable difficulty has been had in obtaining a composition with substantial amounts of aluminum, particularly where the aluminum is to be finely dispersed throughout the lead.
As disclosed in commonly owned pending U.S. Pat. application Ser. No. 249,708, filed Sept. 27, 1988, rapid solidification techniques permit the creation of leadaluminum compositions wherein the aluminum is present in uniformly dispersed regions less than 30 microns in size. However, even when employing this improved method, the melting point of the lead-aluminum mixture (keeping in mind that rapid solidification techniques involve melting the mixture, and then instantly solidifying it before large regions of a given element or compound have a chance to form) rises as the aluminum content increases At high aluminum concentrations, i.e., exceeding about 4%, the high melting point of the mixture caused the lead and aluminum to vaporize and oxidize. Metal vapors, e.g. lead vapor, are environmentally hazardous and must be either eliminated or contained with expensive equipment.
The present invention permits high aluminum content in a lead-aluminum composition without substantial vaporization or oxidation of the lead and aluminum mixture while permitting small uniformly dispersed aluminum regions in the resulting product.
Also, considerable effort has been expended toward the development of lead-antimony battery grids. Lead alloys, typically containing up to 8% or as high as 12% antimony, have been widely used in the construction of automotive batteries for starting-lighting-ignition (SLI) applications. Moreover, grids of such alloys are used where deepdischarge/charge cycles are encountered (for example, in golf carts and fork lift trucks in warehouses). Such alloys are stronger than pure lead and they provide better bonding and electrical contact with the "active material" on the grid. The "active material" is essentially lead sulfate that is electrochemically converted to lead dioxide when the battery is charged. On discharge, the dense lead dioxide reverts to relatively bulky lead sulfate. This "shape change" promotes "paste shedding" from the positive plate. Antimonial lead grids develop an intercrystalline bond with the active material, thereby reducing paste shedding.
However, some antimony from the positive grid migrates through the electrolyte to the negative electrode. This causes a lowering of the hydrogen overvoltage on the surface of the negative electrode in the battery with resulting gassing (hydrogen bubbles form and escape - through decomposition of water in the water/sulfuric acid electrolyte in the battery). As a result, it is necessary to add "makeup water" to replenish that lost through gassing. Otherwise, the battery would fail by drying out.
At least two methods of producing lead-antimony alloy grids for batteries have been employed, namely, casting and rolling. Cast grids are made by pouring molten lead alloy into a mold and then separating the chilled metal from the mold. For example, a "book-mold" may be used to cast battery grids, so that the grid can be removed when the mold is opened. In recent years, equipment has been developed for continuous casting of grids which are subsequently cut apart into individual grids.
To assist in the automation of battery manufacturing, lead-antimony alloy strip can be rolled to the desired grid thickness, slit intermittently in the longitudinal direction, and then expanded sideways to form diamond-shaped openings to accept the active material paste.
In the case of both cast and rolled lead-antimony grids, conventional heat treatment in an oven cannot provide the rapid cooling rates necessary to obtain the desired properties. The resulting relatively coarse grain structure leads to corrosion problems, and, particularly in the case of rolled grids, the lack of mechanical strength is a problem.
In order to overcome the inconvenience, cost, and hazards of adding water to lead-acid batteries, so-called "maintenance-free" batteries were developed some twenty years ago. Such batteries are "sealed" to prevent loss of water (but they do have a safety valve in case the internal pressure should become hazardous). They also contain a "recombination catalyst" which allows the oxygen that was evolved from the positive electrode (during the charging step) to react with the hydrogen gas from the negative electrode to form water, which drips back into the electrolyte. Antimony-free grids are required for the successful operation of maintenance-free batteries (otherwise excessive gassing would occur at the negative electrode, due to antimony migration, and the internal pressure would rise to unacceptable levels).
For use in sealed maintenance-free lead-acid batteries, the requisite antimony-free grids may be made by casting or by rolling strips of lead-calcium type alloys. In one continuous casting process, the lead-calcium alloy is formed into a strip by partially immersing a cooled, rotating drum into the molten alloy and then continuously peeling off the chilled strip. The resulting strip has a finer grain structure on the side next to the rotating drum and a coarser structure on the last-to-cool surface.
To improve the castability and other properties of the lead-calcium alloys, ternary elements (such as tin or aluminum) may be added to the alloys. In any case, the structure and corrosion resistance of such alloys can be improved by enhancing the grain refinement.
Lead-calcium type alloy grids of the expanded metal type can also be produced from rolled strips. In this case, however, the metallurgical structure is the same on top and bottom surfaces of the grid, and may generally be finer than that of a continuously cast grid.
At least three problems are associated with antimony-free battery grids: intergranular corrosion and weakening of the positive grids (due to selective attack of the tin-rich grain boundaries); inadequate bonding of the active material to the grids; and premature loss of battery capacity during deep discharge/charge cycling. It is anticipated that these problems can be solved, at least in part, by the present invention.
Rare earth elements, such as mischmetal (ceriumlanthanum) and neodymium, are more corrosion resistant than tin. Thus, by adding rare earths to lead-calcium type battery grid alloys, the rate of intergranular corrosion could be reduced. By treating a mischmetal-containing grid according to the present invention, the mischmetal can be segregated in the grain boundaries and inhibit selective corrosion of the tin (or aluminum). Also, enhanced grain refinement would distribute the tin more extensively in thinner, more numerous, grain boundaries, thereby further increasing corrosion resistance.